see some of them are just frowny but some of 'em - first two especially - have a special puffed-out lips/cheeks thing goin' on which definitely implies rueful to me, or a more thoughtful kind of regret.

The flehmen response, pronounced [ˈflɛːmən or fleɪmən], also called the flehmen position, flehmen reaction, flehming, or flehmening (from German flehmen, meaning to curl the upper lip), is a particular type of curling of the upper lip in ungulates, felids, and many other mammals, which facilitates the transfer of pheromones and other scents into the vomeronasal organ, also called the Jacobson's Organ.

Bush finished his address to the nation, went back out through the doors, and the doors closed behind him and the national TV broadcast went off the air. And then after the live TV was off, the doors suddenly reopened, and he came back in to say goodbye to all the guests – his former cabinet members, his current cabinet members, Dick Cheney, his daughters, Laura and lots of other friends and supporters. He popped out that door, and when the door opened and he came through it, the look on his face was like no look I’d ever seen on George Bush’s face in my life. I actually flagged three versions as he comes through the door and his expression changes.

And I turned to one of my editors — First I said, “Oh, my God.” And he said, “What?” And I said, “You’ve got to see this picture of Bush. This is really stunning.” And I flipped it over to him to process and his first reaction was, “Wow.” And I said, “If he wasn’t just back there behind that door crying, I don’t know what that look on his face is.” Because he just looks absolutely devastated as he comes through this door after essentially ending his eight year presidency. And it’s just really striking. He just looks absolutely devastated.

some great ones above tho. sometimes I do this expression when I walk out onto the street and it's cold or I'm hungover. also it can be used for when a friend says something sort of bad and you do it and maybe quickly dart your head to the side in place of a "yep". (a bit like the motion that might accompany a wink, if you were a cowboy)